Tennis racket stringing machine

ABSTRACT

Disclosed is a tennis racket stringing machine. The machine comprises racket fastening means including a lower supporting plate on which a head frame of a tennis racket is seated to be securely maintained in a lengthwise direction of the lower supporting plate, a fixed section which is projectedly disposed adjacent to one end of the lower supporting plate and functions to fasten one end of the head frame, a movable section which is disposed adjacent to the other end of the lower supporting plate to be capable of being moved along the lengthwise direction and functions to fasten the other end of the head frame, and a clamp which is located below the lower supporting plate to rotatably support the lower supporting plate; and tensioning force adjusting means separately located from the racket fastening means, for adjusting tensioning force of strings installed through the head frame.

TECHNICAL FIELD

[0001] The present invention relates to a portable tennis racket stringing machine, and more particularly, the present invention relates to a tennis racket stringing machine which enables a user to easily adjust tensioning force of strings installed through a head frame of a tennis racket.

BACKGROUND ART

[0002] Generally, a tennis racket essentially consists of a racket handle, a throat, a head frame and strings. A string which is also called a gut, serves as an energy center of the tennis racket. In other words, a string bed which is formed inside the head frame by installation of the strings through the head frame, is directly brought into contact with a tennis ball to hit the tennis ball. At this time, stroke energy which is generated by a tennis player, is transferred to the tennis ball via the strings.

[0003] As a string for a tennis racket, a natural gut string and a synthetic string are mainly used. In order to allow a tennis racket to be preserved under the best condition, tensioning force of strings must be adjusted to be suited to a particular player. By research materials, it was found that, once a string is installed through a head frame, if four weeks are elapsed, an elasticity loss reaches at the maximum 15% in the case of the natural gut string and 20% in the case of the synthetic string, and if eight hours are elapsed, an elasticity loss reaches at the maximum 5% in the case of the natural gut string and 15% in the case of the synthetic string. Therefore, even though a string did not snap, the string must be changed with new one every three months, or every six months at latest.

[0004] Also, in a tennis racket, it is necessary to finely adjust tensioning force of strings depending upon a situation. That is to say, by finely adjusting tensioning force of strings relying upon a factor such as a season, a temperature, a court surface condition, or the like, it is possible to vary repulsive power of a string bed for a tennis ball.

[0005] Apart from the fact that, although strings of a tennis racket did not snap, the strings must be changed with new ones every predetermined period of time, particularly, in the case of a professional tennis player, snapping of strings frequently occurs. When the strings are made of natural gut strings, the snapping of the strings more frequently occurs. Thus, in order to adjust tensioning force of tennis racket strings so that tensioning force is suited to a particular player, or in order to change a snapped string, a tennis racket stringing machine is needed.

[0006] Referring to FIG. 1, there is shown a schematic perspective view illustrating a conventional tennis racket stringing machine. Since the conventional tennis racket stringing machine 100 must be constructed in such a way as to endure high tensioning force, it cannot but have a large-sized structure. Due to this, it is the norm that the tennis racket stringing machine 100 is provided to a limited place where a tennis racket can be repaired and tensioning force of strings can be adjusted, such as a tennis shop. Hence, a problem is caused in that a tennis player must drop in at the place such as the tennis shop so as to change a string or adjust tensioning force of a string.

[0007] Namely, while tensioning force of strings must be frequently and finely adjusted relying upon a factor such as a temperature, a humidity, a court surface condition, or the like, so as to induce an optimum playing efficiency, inconvenience is provoked in that a tennis racket user must get out of a tennis court and visit a location which is equipped with a tennis racket stringing machine, on all such occasions. This has a time-consuming nature and causes bothersomeness to the tennis racket user.

[0008] Moreover, due to the fact that a diversity of tennis racket stringing machines which are provided to tennis shops, are respectively configured in such a way as to apply different levels of tensioning force to the string, in order to adjust tensioning force of the string so that the tensioning force is optimally suited to the user, a stringing operation must be repeatedly performed through a multitude of times.

DISCLOSURE OF THE INVENTION

[0009] Accordingly, the present invention has been made in an effort to solve the problems occurring in the related art, and an object of the present invention is to provide a tennis racket stringing machine in which respective component elements are fabricated in such a way as to be capable of being assembled one with another and thereby allow the machine to be carried around and conveniently used irrespective of time and place, and which renders a simple configuration with a reduced manufacturing cost.

[0010] Another object of the present invention is to provide a tennis racket stringing machine which enables tensioning force of a tennis racket to be adjusted and thereby the tennis racket to be held under optimum tensioning force suitable for each tennis player, whereby a stable tennis match can be quickly effectuated and reliability of the tennis racket stringing machine can be elevated.

[0011] In order to achieve the above objects, according to one aspect of the present invention, there is provided a tennis racket stringing machine comprising: racket fastening means including a lower supporting plate on which a head frame of a tennis racket is seated and which has a larger length than the head frame so that the head frame of the tennis racket can be securely maintained on the lower supporting plate in a lengthwise direction of the lower supporting plate, a fixed section which is projectedly disposed adjacent to one end of the lower supporting plate and functions to fasten one end of the head frame of the tennis racket, a movable section which is disposed adjacent to the other end of the lower supporting plate in such a way as to be capable of being moved along the lengthwise direction of the lower supporting plate and functions to fasten the other end of the head frame of the tennis racket, and a clamp which is located below the lower supporting plate in such a way as to rotatably support a center portion of the lower supporting plate; and tensioning force adjusting means brought into close contact with the head frame of the tennis racket in a state in which it is separated from the racket fastening means, for adjusting tensioning force of strings installed through the head frame so that the strings can be held under optimum tensioning force.

[0012] According to another aspect of the present invention, the fixed section comprises a fixed shaft which has one end secured to the lower supporting plate and the other end projecting upward, a first upper auxiliary plate which is fitted around the fixed shaft, and a first butterfly nut which is threadedly locked to the other end of the fixed shaft.

[0013] According to another aspect of the present invention, the movable section comprises a pair of bent pieces which are formed by the fact that a pair of portions of the lower supporting plate are partially cut and bent downward, a rotating bolt which is rotatably fitted through the pair of bent pieces in such a way as to be supported by the pair of bent pieces and to a circumferential outer surface of which a manipulating wheel is secured at a predetermined position, a movable body which is threadedly coupled with the rotating bolt so that the movable body can be moved along the lengthwise direction of the lower supporting plate by rotation of the rotating bolt, a movable shaft which has one end secured to an upper surface of the movable body and the other end projecting upward, a second upper auxiliary plate which is fitted around the other end of the movable shaft, and a second butterfly nut which is threadedly locked to the other end of the movable shaft.

[0014] According to another aspect of the present invention, the movable section comprises a pair of guide rails which are formed at the other end of the lower supporting plate, a wheel receiving groove which is defined through the lower supporting plate adjacent to the pair of guide rails, a movable block which is assembled with the pair of guide rails so that the it can be slid along the pair of guide rails in the lengthwise direction of the lower supporting plate, a movable bolt which has one end secured to the movable block and the other end extending in the lengthwise direction of the lower supporting plate in such a way as to be capable of being moved inward of the lower supporting plate, a manipulating wheel which is received in the wheel receiving groove and is threadedly coupled with the movable bolt so that the movable bolt can be moved along the lengthwise direction of the lower supporting plate by rotation of the manipulating wheel, a movable shaft which has one end secured to an upper surface of the movable block and the other end projecting upward, a second upper auxiliary plate which is fitted around the other end of the movable shaft, and a second butterfly nut which is threadedly locked to the other end of the movable shaft.

[0015] According to still another aspect of the present invention, the lower supporting plate is composed of a pair of lower supporting plate halves, a socket portion is defined in a first lower supporting plate half, and a plug portion which is fitted into the socket portion, is formed on a second lower supporting plate half.

[0016] According to yet still another aspect of the present invention, the tensioning force adjusting means comprises a string drawing section, a tensioning force adjusting section and a motor stopping section; the string drawing section including a squeezing plate which is brought into close contact with the head frame of the tennis racket, a squeezing rod which is coupled to the squeezing plate so that it is aligned with a string installed through the head frame, a string reel which winds up the string connected to the squeezing plate, and a motor for rotatably driving the string reel; the tensioning force adjusting section including a seesaw plate which has one end coupled to the squeezing rod so that the seesaw plate can be rotated about a center shaft, a coil spring which has one end secured to the other end of the seesaw plate, and a tensioning force-adjusting dial which is connected to the other end of the coil spring; and the motor stopping section including a projection which is formed on a circumferential outer surface of the squeezing rod at a preselected position, and a limit sensor which is arranged on a moving path of the projection and functions to stop the motor when the projection comes into contact with the limit sensor.

[0017] By the features of the present invention, not only tensioning force of a tennis racket can be finely adjusted to a level which is desired by a user, but also an entire structure of the tennis racket stringing machine is simplified, whereby the machine can be easily carried around and conveniently used irrespective of time and place, and it is possible to manufacture the tennis racket stringing machine with a reduced cost. Portability of the tennis racket stringing machine according to the present invention is further improved by the fact that a lower supporting plate is composed of a pair of lower supporting plate halves.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above objects, and other features and advantages of the present invention will become more apparent after a reading of the following detailed description when taken in conjunction with the drawings, in which:

[0019]FIG. 1 is a schematic perspective view illustrating a conventional tennis racket stringing machine;

[0020]FIG. 2 is an exploded perspective view illustrating an entire structure of a tennis racket stringing machine in accordance with an embodiment of the present invention;

[0021]FIG. 3 is a perspective view illustrating a used status of the tennis racket stringing machine of FIG. 2;

[0022]FIG. 4 is a cross-sectional view illustrating the used status of the tennis racket stringing machine of FIG. 2;

[0023]FIG. 5 is a perspective view illustrating a structure of tensioning force adjusting means which constitutes the tennis racket stringing machine according to the present invention;

[0024]FIG. 6 is an exploded perspective view illustrating an entire structure of a tennis racket stringing machine in accordance with another embodiment of the present invention; and

[0025]FIG. 7 is a cross-sectional view illustrating a used status of the tennis racket stringing machine of FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

[0026] Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts.

[0027]FIG. 2 is an exploded perspective view illustrating an entire structure of a tennis racket stringing machine in accordance with an embodiment of the present invention; FIG. 3 is a perspective view illustrating a used status of the tennis racket stringing machine of FIG. 2; and FIG. 4 is a cross-sectional view illustrating the used status of the tennis racket stringing machine of FIG. 2.

[0028] As shown in the drawings, a tennis racket stringing machine according to a first embodiment of the present invention, which is designated by the drawing reference numeral 10, includes racket fastening means 20 for fastening a head frame 51 of a tennis racket 50 along a lengthwise direction thereof, and tensioning force adjusting means 60 separately disposed from the racket fastening means 20, for holding strings 53 which are installed through the head frame 51, under predetermined tensioning force.

[0029] As shown in FIGS. 2 through 4, the racket fastening means 20 includes a lower supporting plate 21, a fixed section, a movable section and a clamp 27.

[0030] The head frame 51 of the tennis racket 50 is seated on the lower supporting plate 21. The lower supporting plate 21 has a larger length than the head frame 51 so that the lower supporting frame 21 can fasten the head frame 51 of the tennis racket 50 in a lengthwise direction of the lower supporting plate 21.

[0031] The fixed section includes a fixed shaft 23. The fixed shaft 23 has one end which is fastened to an upper surface of the lower supporting plate 21 adjacent to one end of the lower supporting plate 21 and the other end which projects upward. A threaded portion is formed on a circumferential outer surface of the fixed shaft 23. A first upper auxiliary plate 25 is fitted around the other end of the fixed shaft 23, and then, a first butterfly nut 22 is threadedly locked to the other end of the fixed shaft 23. Therefore, in a state wherein the fixed shaft 23 is brought into close contact with one end of the head frame 51 of the tennis racket 50, by tightening the first butterfly nut 22, the first upper auxiliary plate 25 is pressed against the one end of the head frame 51, and thereby, the one end of the head frame 51 can be maintained in a securely fastened status. When the fixed shaft 23 is brought into close contact with the one end of the head frame 51 of the tennis racket 50, in order to prevent the head frame 51 from being damaged by the threaded fixed shaft 23, a first protective ring 23 a is fitted around the fixed shaft 23.

[0032] The movable section is disposed adjacent to the other end of the lower supporting plate 21 in such a way as to be capable of being moved along the lengthwise direction of the lower supporting plate 21 and functions to fasten the other end of the head frame 51 of the tennis racket 50. The movable section includes a pair of bent pieces 37. The pair of bent pieces 37 are formed by the fact that a pair of portions of the lower supporting plate 21 are partially cut and bent downward. A rotating bolt 34 is rotatably fitted through the pair of bent pieces 37 in such a way as to be supported by the pair of bent pieces 37. A manipulating wheel 35 is secured at a predetermined position to a circumferential outer surface of the rotating bolt 34. A movable body 36 is threadedly coupled with the rotating bolt 34. If the is rotating bolt 34 is rotated by the medium of manipulation of the manipulating wheel 35, the movable body 36 can be moved along the lengthwise direction of the lower supporting plate 21. One end of a movable shaft 33 is secured to an upper surface of the movable body 36, and the other end of the movable shaft 33 projects upward. A threaded portion is formed on a circumferential outer surface of the movable shaft 33. A second upper auxiliary plate 25 is fitted around the other end of the movable shaft 33, and then, a second butterfly nut 22 is threadedly locked to the other end of the movable shaft 33. Thus, in a state wherein the movable shaft 33 is brought into close contact with the other end of the head frame 51 of the tennis racket 50, by tightening the second butterfly nut 22, the second upper auxiliary plate 25 is pressed against the other end of the head frame 51, and thereby, the other end of the head frame 51 can be maintained in a securely fastened status. When the movable shaft 33 is brought into close contact with the other end of the head frame 51 of the tennis racket 50, in order to prevent the head frame 51 from being damaged by the threaded movable shaft 33, a second protective ring 33 a is fitted around the movable shaft 33.

[0033] The clamp 27 is located below the lower supporting plate 21 and functions to rotatably support a center portion of the lower supporting plate 21. The clamp 27 includes a rotating knob 26 which is provided at a lower end thereof. A jaw is formed on an upper end of the rotating knob 26. By rotating the rotating knob 26 and thereby moving the jaw upward, the clamp 27 can be firmly affixed to a desired supporting member such as a desk. A pipe into which a height regulating rod 28 is threadedly fitted, is integrally secured to an upper end of the clamp 27. A nut 29 is threadedly coupled to a circumferential outer surface of the height regulating rod 28. Hence, by rotating the nut 29, it is possible to regulate a height of the height regulating rod 28. An upper end of the height regulating rod 28 is fitted into a hole 24 which is defined at the center portion of the lower supporting plate 21. Accordingly, the lower supporting plate 21 can be freely rotated on the height regulating rod 28.

[0034] The tensioning force adjusting means 60 is also brought into close contact with the head frame 51 of the tennis racket 50 in a state wherein it is separated from the racket fastening means 20, and functions to adjust tensioning force of the strings 53 which are installed through the head frame 51, so that the strings 53 can be held under optimum tensioning force. The tensioning force adjusting means 60 includes a string drawing section, a tensioning force adjusting section and a motor stopping section (see FIG. 5). The string drawing section functions to draw the strings 53 through the head frame 51 of the tennis racket 50. The string drawing section includes a squeezing plate 61 which is brought into close contact with the head frame 51 of the tennis racket 50, a squeezing rod 62 which is coupled to the squeezing plate 61 so that it is aligned with a string 53 installed through the head frame 51, a string reel 63 which winds up the string 53 connected to the squeezing plate 61, and a motor 64 for rotatably driving the string reel 63.

[0035] The tensioning force adjusting section includes a seesaw plate 65 which has one end coupled to the squeezing rod 62 so that the seesaw plate 65 can be rotated about a center shaft 70, a coil spring 66 which has one end secured to the other end of the seesaw plate 65, and a tensioning force adjusting dial 67 which is connected to the other end of the coil spring 66. An indicating pin 67 a is provided on the tensioning force adjusting dial 67, and functions to indicate current tensioning force on a scale mark portion 72 which is given on an outer surface of the tensioning force adjusting means 60. In order to allow force to be stably transferred to respective surfaces of the squeezing rod 62 and the coil spring 66 which surfaces are brought into contact with both ends of the seesaw plate 65 which are installed so as to be capable of being rotated about the center shaft 70, ball members (not shown) are properly arranged on the surfaces, respectively.

[0036] The motor stopping section functions to stop the motor 64 when the strings 53 are held under optimum tensioning force. The motor stopping section includes a projection 68 which is formed on a circumferential outer surface of the squeezing rod 62 at a preselected position, and a limit sensor 69 which is arranged on a moving path of the projection 68 and functions to stop the motor 64 when the projection 68 comes into contact with the limit sensor 69. Here, it is preferred that the motor stopping section includes a reset button 71. In this regard, the reset button 71 can rotate the motor 64 by one revolution in a reverse direction so that the string 53 which is wound on the string reel 63, can be smoothly unwound therefrom. The drawing reference numeral 57 represents a string clamp.

[0037] Hereinafter, operations of the tennis racket stringing machine 10 in accordance with the first embodiment of the present invention, constructed as mentioned above, will be described in detail with reference to FIGS. 3 and 4.

[0038] When it is necessary to change the string 53 of the tennis racket 50 or adjust tensioning force of the string 53, a user first affixes the clamp 27 to a desired supporting member such as a desk. At this time, if a racket installation height of the stringing machine 10 does not correspond to a center portion of the tennis racket 50, that is, the tennis racket stringing machine 10 does not exist on the same plane as the center portion of the tennis racket 50, by rotating the nut 29 of the clamp 27 and thereby causing movement of the height regulating rod 28 in upward and downward directions, a fastening height for the tennis racket 50 can be adequately regulated.

[0039] Then, the head frame 51 of the tennis racket 50 is fastened to the lower supporting plate 21. In other words, in a state wherein the first and second protective rings 23 a and 33 a are respectively fitted around the fixed shaft 23 and the movable shaft 33 so as to prevent the head frame 51 from being damaged by the threaded fixed shaft 23 and the threaded movable shaft 33 when the fixed shaft 23 and the movable shaft 33 are brought into contact with the head frame 51, the one end and the other end of the head frame 51 are positioned on the fixed section and the movable section of the racket fastening means 20, respectively. At this time, position alignment between the other end of the head frame 51 and the movable section of the racket fastening means 20 is effected by the manipulation of the manipulating wheel 35 of the rotating bolt 34 and positional shift of the movable shaft 33 thereby. Thereafter, by tightening the first and second butterfly nuts 22, the first and second upper auxiliary plates 25 press the head frame 51 against the lower supporting plate 21, and thereby the head frame 51 is maintained in the securely fastened status. By this, the head frame 51 of the tennis racket 50 is supported in a manner such that the strings 53 can be installed therethrough.

[0040] A string changing method is commonly applied to all tennis rackets, and therefore, main strings and cross strings must be knotted in sheared statuses. That is to say, four knots must be formed, and the main string and the cross string should be knotted while they are drawn one by one by the string reel 63. As a consequence, the user can wind the main string and the cross string in regular sequence on the string reel 63 which draws the strings 53 with the predetermined tensioning force, and then, can knot the main string and the cross string with each other while clamping first wound string 53 using the string clamp 57 in such a way as not to allow the first wound string 53 to be unwound from the string reel 63. Since the lower supporting plate 21 is configured in a manner such that it can be rotated on the clamp 27, the main strings and the cross strings can be wound on the string reel 63 while being rotated.

[0041]FIG. 6 is an exploded perspective view illustrating an entire structure of a tennis racket stringing machine in accordance with another embodiment of the present invention; and FIG. 7 is a cross-sectional view illustrating a used status of the tennis racket stringing machine of FIG. 6.

[0042] In a tennis racket stringing machine in accordance with this second embodiment of the present invention, only a structure of racket fastening means is partially differentiated from that of the racket fastening means 20 in accordance with the first embodiment of the present invention. Thus, only the differences existing therebetween will be described hereinbelow.

[0043] In this embodiment of the present invention, a lower supporting plate 11 is composed of a pair of lower supporting plate halves 73 and 74. A socket portion 75 is defined in a first lower supporting plate half 73, and a plug portion 76 which is fitted into the socket portion 75, is formed on a second lower supporting plate half 74.

[0044] A pair of guide rails 12 are formed at one end of the second lower supporting plate half 74. Adjacent to the pair of guide rails 12, a wheel receiving groove 13 is defined through the second lower supporting plate half 74. Adjacent to the wheel receiving groove 13, an accommodating slot 11 a is defined through the second lower supporting plate half 74. A movable section of the racket fastening means according to this embodiment of the present invention includes a movable block 14. The movable block 14 is assembled with the pair of guide rails 12 so that it can be slid along the pair of guide rails 12 in the lengthwise direction of the lower supporting plate 11. Both sides of the movable block 14 are respectively defined with a pair of guide grooves so that the pair of guide rails 12 can be engaged into the pair of guide grooves, respectively. A movable bolt 17 is secured at one end thereof to the movable block 14. The other end of the movable bolt 17 movably projects into the accommodating slot 11 a through the wheel receiving groove 13 which is defined through the second lower supporting plate half 74. A manipulating wheel 16 is received in the wheel receiving groove 13. The manipulating wheel 16 is threadedly coupled with the movable bolt 17 so that the movable bolt 17 can be moved along the lengthwise direction of the lower supporting plate 11 by the medium of rotation of the manipulating wheel 16. One end of a movable shaft 15 is secured to an upper surface of the movable block 14. The other end of the movable shaft 15 projects upward. A threaded portion is formed on a circumferential outer surface of the movable shaft 15. A second upper auxiliary plate 18 is fitted around the other end of the movable shaft 15, and then, a second butterfly nut 22 is threadedly locked to the other end of the movable shaft 15. Hence, in a state wherein the movable shaft 15 is brought into close contact with an end of the head frame 51 of the tennis racket 50, by tightening the second butterfly nut 22, the second upper auxiliary plate 18 is pressed against the end of the head frame 51, and thereby, the end of the head frame 51 can be maintained in a securely fastened status. When the movable shaft 15 is brought into close contact with the end of the head frame 51 of the tennis racket 50, in order to prevent the head frame 51 from being damaged by the threaded movable shaft 15, a second protective ring 15 a is fitted around the movable shaft 15.

[0045] Because it is sufficient for the movable shaft 15 to function to securely fasten the head frame 51 between the lower supporting plate 11 and the second upper auxiliary plate 18, instead of the fact that the one end of the movable shaft 15 is secured to the upper surface of the movable block 14 as shown in FIG. 6, the movable shaft 15 can be shaped in the form of a shaft which is integrally secured to the second butterfly nut 22 as shown in FIG. 7. Of course, in this case, it is to be readily understood that a threaded hole into which the shaft can be threadedly fitted, must be defined in the movable block 14.

[0046] Consequently, as a user rotates the manipulating wheel 16 and thereby causes the movable block 14 to be moved along the lengthwise direction of the lower supporting plate 11, the head frame 51 of the tennis racket 50 can be easily fastened to the lower supporting plate 11.

INDUSTRIAL APPLICABILITY

[0047] As a result, the tennis racket stringing machine according to the present invention provides advantages in that, since tensioning force of a tennis racket can be finely adjusted to a level which is desired by a user, and an entire structure of the tennis racket stringing machine is simplified, the machine can be easily carried around and conveniently used irrespective of time and place, and it is possible to manufacture the tennis racket stringing machine with a reduced cost. Portability of the tennis racket stringing machine according to the present invention is further improved by the fact that a lower supporting plate is composed of a pair of lower supporting plate halves.

[0048] In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims. 

1. A tennis racket stringing machine comprising: racket fastening means including a lower supporting plate on which a head frame of a tennis racket is seated and which has a larger length than the head frame so that the head frame of the tennis racket can be securely maintained on the lower supporting plate in a lengthwise direction of the lower supporting plate, a fixed section which is projectedly disposed adjacent to one end of the lower supporting plate and functions to fasten one end of the head frame of the tennis racket, a movable section which is disposed adjacent to the other end of the lower supporting plate in such a way as to be capable of being moved along the lengthwise direction of the lower supporting plate and functions to fasten the other end of the head frame of the tennis racket, and a clamp which is located below the lower supporting plate in such a way as to rotatably support a center portion of the lower supporting plate; and tensioning force adjusting means brought into close contact with the head frame of the tennis racket in a state in which it is separated from the racket fastening means, for adjusting tensioning force of strings installed through the head frame so that the strings can be held under optimum tensioning force.
 2. The tennis racket stringing machine as claimed in claim 1, wherein the fixed section comprises a fixed shaft which has one end secured to the lower supporting plate and the other end projecting upward, a first upper auxiliary plate which is fitted around the fixed shaft, and a first butterfly nut which is threadedly locked to the other end of the fixed shaft.
 3. The tennis racket stringing machine as claimed in claims 1 or 2, wherein the movable section comprises a pair of bent pieces which are formed by the fact that a pair of portions of the lower supporting plate are partially cut and bent downward, a rotating bolt which is rotatably fitted through the pair of bent pieces in such a way as to be supported by the pair of bent pieces and to a circumferential outer surface of which a manipulating wheel is secured at a predetermined position, a movable body which is threadedly coupled with the rotating bolt so that the movable body can be moved along the lengthwise direction of the lower supporting plate by rotation of the rotating bolt, a movable shaft which has one end secured to an upper surface of the movable body and the other end projecting upward, a second upper auxiliary plate which is fitted around the other end of the movable shaft, and a second butterfly nut which is threadedly locked to the other end of the movable shaft.
 4. The tennis racket stringing machine as claimed in claims 1 or 2, wherein the movable section comprises a pair of guide rails which are formed at the other end of the lower supporting plate, a wheel receiving groove which is defined through the lower supporting plate adjacent to the pair of guide rails, a movable block which is assembled with the pair of guide rails so that the it can be slid along the pair of guide rails in the lengthwise direction of the lower supporting plate, a movable bolt which has one end secured to the movable block and the other end extending in the lengthwise direction of the lower supporting plate in such a way as to be capable of being moved inward of the lower supporting plate, a manipulating wheel which is received in the wheel receiving groove and is threadedly coupled with the movable bolt so that the movable bolt can be moved along the lengthwise direction of the lower supporting plate by rotation of the manipulating wheel, a movable shaft which has one end secured to an upper surface of the movable block and the other end projecting upward, a second upper auxiliary plate which is fitted around the other end of the movable shaft, and a second butterfly nut which is threadedly locked to the other end of the movable shaft.
 5. The tennis racket stringing machine as claimed in claim 4, wherein the lower supporting plate is composed of a pair of lower supporting plate halves, a socket portion is defined in a first lower supporting plate half, and a plug portion which is fitted into the socket portion, is formed on a second lower supporting plate half.
 6. The tennis racket stringing machine as claimed in claim 1, wherein the tensioning force adjusting means comprises a string drawing section, a tensioning force adjusting section and a motor stopping section; the string drawing section including a squeezing plate which is brought into close contact with the head frame of the tennis racket, a squeezing rod which is coupled to the squeezing plate so that it is aligned with a string installed through the head frame, a string reel which winds up the string connected to the squeezing plate, and a motor for rotatably driving the string reel; the tensioning force adjusting section including a seesaw plate which has one end coupled to the squeezing rod so that the seesaw plate can be rotated about a center shaft, a coil spring which has one end secured to the other end of the seesaw plate, and a tensioning force adjusting dial which is connected to the other end of the coil spring; and the motor stopping section including a projection which is formed on a circumferential outer surface of the squeezing rod at a preselected position, and a limit sensor which is arranged on a moving path of the projection and functions to stop the motor when the projection comes into contact with the limit sensor. 